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Number JV53. 

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Accession No. \SA"^S*. 




Cucumber Leaf infested with Whitefiy. 



THE 



WHITE-FLY OF GREENHOUSES 



BY 



CLARENCE M. WEED AND ALBERT F. CONRADI 



NEW HAMPSHIRE COLLEGE 
AGRICULTURAL EXPERIMENT STATION 



DURHAM 



Bulletin loo 



March, 1903 



NEW HAMPSHIRE COLLEGE 



OF 



AGRICULTURE AND THE MECHANIC ARTS 



AGRICULTURAL EXPERIMENT STATION 

DURHAM, N. H. 



BOARD OF CONTROL v 

Hon. JOHN G. TALLANT, Chairman, Pembroke. 
Hon. GEORGE A. VVASON, New Boston. 
CHARLES W. STONE, A. M., Secretary, Andover. 
Pres. CHARLES S. MURKLAND, ex-officio, Durham. 

THE STATION COUNCIL 

FRED W. MORSE, M. S., Vice- Director and Chemist, 
CHARLES H. PETTEE, A. M., C. E., Meteorologist. 
HERBERT H. LAMSON, M. D., Bacteriologist. 
CLARENCE M. WEED, D. Sc, Entomologist. 
FRANK WM. RANE, B. Agr., M. S., Horticulturist, 
*HARRY HAYWARD, M. S., Agriculturist. 

ASSISTANTS 

^ HARRY F. HALL, Assistant Horticulturist. 

ALBERT F. CONRADI, M. S., Assistant Entomologist. 
LUCIAN A. HILL, A. B., Assistant Chemist. 
FREDERICK C. KEITH, Clerk. 

•Resigned March 12, 1903. 



THE WHITE-FLY OF GREENHOUSES 



BY CLARENCE M. WEED AND ALBERT F. CONRADI 



During recent years greenhouse crops in this and other states 
have been very commonly attacked by a tiny insect which has 
received the common name of white-tly. It is a member of a 
small and generally little-known famil}^ whose technical name 
is Aleurodes. It accomplishes its injury by sucking the sap of 
its food-plants, often appearing upon the leaves in enormous 
numbers. A great variety of greenhouse crops suffer from its 
depredations, although in New Hampshire cucumbers and 
tomatoes growing under glass have been damaged the most. 

The white-flies as a family are tropical insects. In warm 
climates man}" species are abundant out of doors, but in cold 
regions onh^ a few sorts appear to be able to survive from sea- 
son to season, except in the shelter of the heated greenhouse. 

LIFE HISTORY 

The egg of the white-fly common in our greenhouses is a 
minute oblong object slightly pointed at one end. The rounded 
end is attached to the under surface of the leaf so that the egg 
hangs perpendicularly downward when the leaf is horizontal. 
Its length is less than 1-4 mm. or about 1-100 inch. When 
first laid the eggs are slightly greenish yellow, but they soon 
become darker. Just before hatching the color is a dull o:lossv 
brown. 

In a little less than two weeks (13 days according to our 
observations) the eggs hatch into little nymphs, which move 
about over the surface of the leaf for a few hours, perhaps a 
day ; then they insert their tiny beaks into the succulent tissues 
of the leaf and settle down, taking on the appearance of a 
minute scale. 

The little scale remains in position two weeks or longer ; 
then it is a tiny flattened object, with oval outline, of a slightly 
greenish-yellow color, in size about 1 mm. by 1-2 mm. Under a 



48 THE WHITE-FLY OF GREENHOUSES 

hand lens one can see that it is scantily clothed with minute 
bristles, having a denser fringe along the margins of the body. 
Finally a T-shaped rupture appears in the back skin of the 
insect, and the adult white-fly emerges to continue the prop- 
agation of the race. 

INJURIES TO THE PLANT 

The mouth parts of both the nymphs and adults of the white- 
fly are formed for sucking, and the insects are injurious in both 
these stages. They insert their tiny beaks into the succulent 
tissues of the under surface of the leaf; the latter soon shrivels, 
dies, and later falls to the ground. As the adults prefer to 
oviposit upon the younger shoots the lower leaves of infested 
plants die first. 

Observations and experiments carried on during the past 
season showed that much the most harm was caused by the 
work of the nymphs. Adults kept in breeding cages with 
plants lived a longer time than those without food, yet in no 
case did the plants die where adults alone were present. 

Some time after the leaves are infested the under surface is 
covered with a sticky substance excreted by the insects. This 
not only tends to close the pores of the leaf, but it supports a 
black fungus which soon covers the entire under surface of 
the leaf, making it appear as though it were covered with soot. 
In a badly infested greenhouse the upper surface shows this 
fungus, but less so than the under surface. 

HIBERNATION AND BROODS 

We have no evidence in our New Hampshire studies that the 
white-flies can pass the winter out of doors. On those plants 
that were infested in the open last summer and since brought 
into the greenhouse, none of its life-history stages can be found. 
The treatment given those in the greenhouses subdued their 
numbers and at the approach of cold weather the few that 
remained were less active. Some adults linger among the 
leaves of yourg beets in the greenhouse during the colder wea- 
ther, occasionally appearing on young tomato plants. Adults 
captured on January 14 and put in vials containing no food 
died in three days, others kept in bell jars with plants survived. 
No nymphs could be found at this time, and it is probable that 



REMEDIAL MEASURES 49 

those few found in tlie house at this time of the year are adults 
that escaped the summer treatment. 

The time required for this insect to complete all its life 
histor}' stages is not more than five weeks, allowing one week's 
time for oviposition. It follows that an indefinite number of 
broods are possible, and if left unchecked their numbers may 
easily become most seriously destructive. 

Although this species of white-fly is a serious pest on to- 
matoes and cucumbers it is not confined to these plants, nor is 
it confined to forcing houses alone. On July 18, an immense 
swarm was discovered in the gardens out of doors. A careful 
investigation was made to discover nymphs on as many plants 
as possible. They were found on a stray potato plant and 
strawberry leaves. On July 26, some were found on 
currant leaves, also on the leaves of a species of Erigeron. 
On July 29, some were found on goldenrod, and a little later 
a nymph was found on a chrysanthemum leaf. Of course, 
these were very few and in all cases on the under surface of 
the leaf. Unfortunately it has not been determined whether or 
not these nymphs complete their life history successfully. 

Mr. W. E. Britton of the Connecticut station gives a list of 
58 plants upon which he found the nymphs. It follows that 
this pest has a wide range of adaptation in food plants. In 
our studies we have no evidence of its establishing itself in the 
open in a serious way. 

REMEDIAL MEASURES 

Kerowater Sprays. Our experiments during the summer of 
1901 showed that the adult white-flies are very easily killed by 
spraying with a mechanical mixture of kerosene and water, 
with 5 per cent, of kerosene. We used for this purpose a knap- 
sack kerowater sprayer. When the plant on which they are 
resting is disturbed these adults fly into the air, making a 
miniature cloud. If those thus in the air are hit by the kero- 
water spray they drop down and die. In spraying it is desir- 
able to begin at the top of the plants and work down. 

We used this method to advantage out of doors, the adults 
being very easily destroyed even on badly infested plants. 

The extent to which the kerowater spray can be employed 
to destroy the nymphs will depend largely on the plant attacked. 
Some greenhouse plants are very easily injured by kerosene. 



50 THE WHITE-FLY OF GREENHOUSES 

Our experiments showed that the nymphs are readily killed by 
the kerosene spray. Doubtless kerosene emulsion could be 
used to equal advantage. 

Hydrocyanic Acid Gas. This was the most successful, as 
well as the most satisfactory, remedy we tried. Statements 
had been made that this gas could not be used to advantage 
without injury to the greenhouse crops. To determine the 
truth of this a considerable number of tests were made, the 
results showing that the adult white-flies are very easily killed 
by an exposure too short to injure the plants. The more 
important of these experiments are recorded below : 

On July 15, 1902, a vacant section of the greenhouse con- 
taining 2,833 cubic feet of space was stocked with tomato and 
squash plants and a swarm of white-flies. The gas treatment 
was given at 10 a. m., the day being clear and hot. Twelve 
ounces of strong sulphuric acid and 12 ounces of potassium 
cyanide were added to one gallon of water in an earthenware 
jar. The house was kept closed for 15 minutes, at the end of 
which period the adult white-flies were dead. The plants were 
uninjured, except a few leaves of a lily that had been growing 
under a greenhouse bench. In later experiments it was found 
that the white-flies began to fall off the plants within two min- 
utes after the gas was formed. 

Later, during the same day, the main section of the house 
was treated. This section was filled with fruiting tomatoes 
and cucumbers on which the white-flies were exceedinglv 
abundant and destructive. At 4:30 p. m. the gas was made 
by adding 6 ounces of potassium cyanide to 1 pound of water 
to which 1 pound of strong sulphuric acid had been added. 
Nine minutes later the house was opened, when the earth and 
floors were seen to be covered by myriads of the tiny flies that 
had succumbed to the deadly gas. Apparently all the adult 
flies were killed. No plants were injured. 

On July 28 and August 12 this treatment was repeated with 
equally successful results. 

It seems evident from these experiments that the white-fly 
can be controlled in greenhouses by this gas treatment. The 
frequency of treatment and the number of treatments required 
to subdue the pests in a badly infested house will need to be 
determined by further observations. 



A - \ \ 

» . ^, .■ 

HOW TO ESTI3IATE THE CUBIC SPACE IN A HOUSE 51 

HOW TO FUMIGATE 

Before fumigating it is necessary to know the amount of 
space contained in the house ; this will enable the operator to 
mix his chemicals properly. Hydrocyanic acid gas if used too 
strong, or if left in the house too long, will seriously injure 
the plants. It is therefore very important that the directions 
for treatment be followed carefully. 

The best proportion of cyanide, sulphuric acid, and water 
seems to be 

1 ounce cyanide of potassium, 

2 ounces commercial sulphuric acid, 
4 ounces water. 

The water should be poured into the receiving vessel first ; 
then the acid should be added ; then the cyanide as directed 
more fully below. Pour the acid into the icater ; never pour 
the water into the acid. 

In our experiments we used 1 ounce of cyanide to 400 cubic 
feet of space, and left the house closed but nine minutes. Do 
not expose the jjlants to the gas longer than this. 

HOW TO ESTIMATE THE CUBIC SPACE IN A HOUSE 

If the house is the shape of a square or a rectangle the 
cubic contents are found by multiplying the height, length, and 
width. If the house has a triangular roof with two sides equal, 
first find the cubic contents of the space enclosed by the walls 
as directed above ; then find the area of the space enclosed by 
the roof by multiplying the width of the house by the length of 
the house and this result by the perpendicular distance from 
the top of the walls to the gable ; divide this entire product 
by 3, and add the result to that of the space enclosed by the 
walls. If, however, the house is irregularly shaped the prob- 
lem becomes more difficult. The cubic contents may be ob- 
tained in two wavs : 

a. Divide the house into rectangles, squares, and right 
angled triangles. The cubic contents of rectangles and 
squares are obtained by multiplying together the three dimen- 
sions ; if a right angled triangle by multiplying the two shorter 
sides together and divide by 2. 

h. The following method, originated by Dr. B. T. Galloway, 
is quoted from Prof. W. G. Johnson's " Fumigation Methods '* : 

Procure from a stationer a sheet of cross section paper con- 



52 THE WHITE-FLY OF GREENHOUSES 

taining squares of 1-4 aud 1-16 inches. Let the 1-4 inch 
squares represent square feet. Draw on this sheet a cross 
section of your house, on the scale of 1-4 inch to the foot. 
Count the enclosed squares, reduce them to feet, and multiply 
by the length of the house. In this manner the exact cubic 
contents of any house may be easily found no matter how 
irregular its dimensions. 

HOW TO MAKE THE GAS 

The amount of cyanide used in these experiments was at 
the rate of 1 ounce of cyanide of potassium to 400 cubic feet 
of greenhouse space. 

Use an earthen pot ; do not use metal ; this vessel should be 
large enough so that the liquid does not run over on to the 
greenhouse floor. In our experiments an earthenware gallon 
jar was used, and this was not any too large. 

Pour the water into the vessel, add the acid ; then weigh 
the cyanide, put it in a paper bag. Close the ventilators, and 
firmly fasten every door of the house except the one used by 
the operator. This caution should not be neglected, as the 
writers have repeatedly witnessed where a person unawares 
approached and entered the greenhouse when filled with 
tobacco fumes. Were this to hapi)en when the house is filled 
with the hydrocyanic acid gas treatment, the result would be 
serious, if not fatal. When all is ready, approach the pot 
with the bag containing the cyanide ; fill the lungs with air, 
hold your breath, drop in the cyanide, bag aud all, and leave 
the house at once, lock the last door and notice the time. 
After the number of minutes required to kill the adult white- 
flies are passed open the doors and give the gas plenty of time 
to escape before you enter. 

Hydrocyanic acid gas is very poisonous, and the house 
should not be entered during treatment. 

These experiments related to tomatoes and cucunihei^s under 
glass. We do not know what effect the gas might have o?i other 
plants and advise 2^^eliini7iary trials before it is used. 

Greenhouses used for growing vegetables can often be given 
a stronger treatment at times when a crop is matured and the 
house is empty. At such times the gas may be generated in a 
much larger quantity and left over night. It will thus be 
pretty sure to kill all insects present. 



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